1. Field of the Invention
The present invention relates to an object lens driving device used in an optical information recording and/or reproduction apparatus for recording information to and/or reproducing information from a disk-shaped recording medium.
2. Description of the Related Art
If a disk-shaped recording medium (hereinafter, simply referred to as "disk") is warped, the distance between the surface of the recording medium and an object lens of the object lens driving device varies as the recording medium rotates, resulting in a focusing error. If the rotation of the recording medium is off-center, a tracking error occurs. In order to prevent a focusing or tracking error, an object lens driving device controls the object lens to be driven in two directions, namely, along the optical axis of the object lens which is vertical to the surface of the recording medium (focusing direction) and along the direction parallel to the surface of the recording medium (tracking direction).
In the optical information recording and/or reproduction apparatus incorporating the object lens driving device as described above, a relative tilt of the optical axis of the object lens to the surface of a disk (hereinafter, simply referred to as "tilt") may occur besides the focusing error and the tracking error. The tilt is responsible for optical aberration which deteriorates signals during recording and reproduction.
Some conventional optical information recording and/or reproduction apparatus have been proposed for solving the above problems. For example, Japanese Laid-Open Publication No. 9-22537 discloses that the tilt is corrected by providing at least one permanent magnet adhered to a movable section and at least two focusing coils attached on a base and regulating currents flowing through the coils.
Such a conventional optical information recording and/or reproduction apparatus will be described below with reference to the accompanying drawings. FIG. 11 is a perspective view of a structure of the conventional optical information recording and/or reproduction apparatus 500. FIG. 12 is a diagram for explaining the definitions of the reference symbols of the present specification.
Referring to FIG. 11, the apparatus 500 includes an object lens 101, a lens holder 102 for holding the object lens 101, permanent magnets 103a and 103b adhered to the lens holder 102, suspension wires 104, opposed yokes 105a-105d, tracking coils 106a-106d, focusing coils 107a-107d, a suspension holder 108, and a fixation base 109. The object lens 101, the lens holder 102, and the permanent magnets 103a and 103b constitute a movable section 550. A first end of each suspension wire 104 is attached to the movable section 550 while one end of each suspension wire 104 is attached to the suspension holder 108.
Referring to FIG. 12, moving directions of the movable section 550 are defined. In FIG. 12, Fo indicates a focusing direction parallel to an optical axis; Tr a tracking direction perpendicular to the direction Fo; Rt a radial tilt which is a tilt around the axis of a tangential direction; and Tt a tangential tilt which is a tilt around the axis of the tracking direction.
Now the operation of the conventional object lens driving apparatus 500 will be described with reference to FIG. 11. The movable section 550 is driven toward the tracking direction Tr by electromagnetic forces generated by electric currents through the tracking coils 106a-106d traversing in a direction perpendicular to the magnetic flux of the permanent magnets 103a and 103b. Since the tracking coils 106a-106d are fixed on the base 109, the movable section 550 performs its relative substantially translational movement.
The movable section 550 is also driven toward the focusing direction Fo by electromagnetic forces generated by electric currents through the focusing coils 107a-107d traversing in a direction perpendicular to the magnetic flux of the permanent magnets 103a and 103b, performing its substantially translational movement.
Furthermore, the movable section 550 is driven along the direction of the radial tilt Rt by the moment Mr of a force around the Y axis produced by applying to the movable section 550 a driving force in the direction Fo by the focusing coils 107a and 107c and a driving force in the direction Fo, but in the opposite direction, by the focusing coils 107b and 107d. According to this, it is possible to correct the radial tilt.
To enhance the recording capacity of an optical information recording and/or reproduction apparatus using disks, a condensed light spot used for recording and reproducing information to and from the disk has been increasingly made narrower by adopting an object lens having a higher aperture ratio. In this case, the optical aberration caused by the relative tilt of the optical axis of the object lens to the surface of the disk increases in proportion to the third power of the aperture ratio. To obtain satisfactory recording and reproduction signals, it is therefore required to correct the tilt of the optical axis of the object lens to the disk.
Although the above-described structure makes it possible to correct a tilt in a radial direction caused by the warp of a disk and the like, so-called radial tilt, it is difficult to correct a tilt in a tangential direction caused by the bend of a disk, so-called tangential tilt.
The present invention is provided to solve the above problems with the conventional object lens driving device.